Focus on: Stefania Bufalino

A.M First of all I would like to ask you about your current research activities and how they are related to your INFN fellowship?

S.B. I am a post-doc from INFN (division of Torino) and I have recently moved to CERN as I got a fellowship from INFN. I am currently spending my time both on data analysis but also in doing some technical work. Particularly, I am working in the Strangeness Working Group and I am involved in the study of hyper-nuclei and anti-hypernuclei, related to the production of matter and antimatter with strangeness content and trying to find out the role that strangeness plays in heavy ion collisions. In addition I am also involved in the study of Particle Identification capabilities of a new ALICE Inner Tracking System in view of its possible upgrade. These are the two main fields in which I am involved. Moreover, for next year I am also getting involved with another hardware issue, specifically the maintenance of SDD (Silicon Drift Detector of the ITS) and I will be one of the two experts on call. The role of the expert on-call implies the prompt resolution of the problems that happen during the data taking and in particular this role becomes really crucial during the short p-Pb run in which the uptime efficiency is mandatory.

Perhaps I should also add that from May and till the end of August I am the System Run Coordinator for the ITS. I must say that this is a particularly useful experience through which I am learning many things about the detector and the data taking, things that you usually don’t get to know if you are doing only data analysis. It is a great experience!

A.M: So why ALICE? I mean which was your previous trajectory and how have you decided to get involved with the ALICE experiment?

S.B. Well! First, of all, I should probably mention that my PhD thesis wasn’t in heavy ion physics. I was involved in an experiment that was studying the production of Λ hypernuclei. It was the FINUDA experiment and it was based in Frascati in Italy. At some point they decided to stop the experiment and at that point the most reasonable thing for me was to look for another experiment that would study the production of hypernuclei using heavy ion collisions. Therefore, for me ALICE was the best way to continue this kind of research since ion collisions is a good and very effective method of producing hyper-nuclei although it’s based on a different mechanism compared to the one that was used in Frascati.

However, ALICE gives us the possibility of studying not only hyper-nuclei but also anti-hypernuclei which means that we get a 3rd dimension compared to normal nuclei (i.e. consisting of neutrons and protons) and this dimension is related to the parameter of strangeness. In addition, while during the “normal” and well-known methods of producing hypernuclei we are getting negative strangeness, ALICE is allowing us to get positive strangeness.

A.M. Well that sounds quite fascinating. But I guess that first we need to understand better what is a hypernucleus before discussing the importance of strangeness and the ALICE experiment.

"...the introduction of one (or more) hyperons in a nucleus may give rise to various changes of the nuclear structure. For example it can change the size and the shape of the nucleus or the cluster structure. In addition it can lead to the manifestation of new symmetries while also changes the collective motions."

S.B. So, a hypernucleus is a nucleus in which at least one nucleon (proton or neutron) has been replaced by a hyperon. In fact the first hypernucleus was given to us by nature and it was discovered by Danysz and Pniewski in 1952. It was formed in a cosmic ray interaction in a balloon-flown emulsion plate. This was a discovery coming 5 years after the discovery of the first strange particle in 1947 and I should also remind you that it was only one year later that Gell-Mann introduced the concept of strangeness in his QCD algebra. These were all discoveries that allowed us to describe better hypernuclei and better appreciate their importance.

Now, the introduction of one (or more) hyperons in a nucleus may give rise to various changes of the nuclear structure. For example it can change the size and the shape of the nucleus or the cluster structure. In addition it can lead to the manifestation of new symmetries while also changes the collective motions. The reason is that a hyperon is a particle with a different strangeness quantum number and so it can share space and momentum coordinates with the usual nucleons that can differ from each other in spin and isospin.

?.?. And how many hypernuclei have been studied until now?

S.B. As I mentioned, hypernuclei have negative strangeness. We have studied around 40 systems of ?-hypernuclei with S=-1 and only one ?-hypernucleus with the same value of strangeness. Moreover, if we move to strangeness of S=-2 we have only 6 ?? candidate events in emulsions while there has never been observed a ?-hypernucleus. Moreover there is the hypothesis that in the core of neutron stars or stars with some other kind of strange hadronic matter there are hypernuclei with negative strangeness that tends to infinity.

3-D chart of the nuclides. The familiar Periodic Table arranges the elements according to their atomic number, Z, which determines the chemical properties of each element. Physicists are also concerned with the N axis, which gives the number of neutrons in the nucleus. The third axis represents strangeness, S, which is zero for all naturally occurring matter, but could be non-zero in the core of collapsed stars.

A.M. And if I am right hypernuclei can tell us a lot about strangeness? But why is strangeness important in nuclear physics?

S.B. Structure of baryons in nuclear medium and structure of nuclei as baryonic many-body systems can be better studied by introducing a strangeness degree of freedom into a nucleus. The ?-particle lies deep inside a nucleus as it is not limited by the Pauli blocking and hence can penetrate into the nucleus and form deeply bound hypernuclear states. It sits there as an impurity and can carry valuable information about the internal structure of the nucleus. Moreover, while in the case of non strange nuclei the single particle strength is fragmented with excitation energy and the deeply bound hole-states are essentially unobservable this is not the case with hypernuclei where the distinguishable ? may occupy any orbital leading to well defined sharp set of states. This gives us a practical way of studying ?? strong interactions by using spectroscopy and also weak (decay) interaction.

"Equilibration between the strange quark flavour and light quark flavours is one of the proposed signatures of QGP formation, which would result in high anti-hypernucleus yields. In addition, recent theoretical studies motivate a search for the onset of QGP by studying the evolution of the baryon – strangeness correlation as a function of the collision energy"

Having mentioned that, I should add that the main purpose of hypernuclear spectroscopy is to understand better the interactions among baryons. Although NN interaction is well known from elastic scattering data we know very few things about YN (hyperon-nucleon) and YY (hyperon-hyperon) interactions as they involve particles with short lifetimes and there are few scattering data.

Chart of known Λ hypernuclei in the (N,Z) plane: the different colours refers to the different experimental methods used to produce the hypernuclei reported in the chart.

A.M. So what is so special with ALICE and how does it contribute to the study of hypernuclei?

S.B. The measurement of the hypernuclei/anti-hypernuclei production opens up a very interesting new regime for test of particle production at chemical equilibrium. In fact, the experimental determination of the yield ratio between anti-hypernuclei and hypernuclei is a powerful tool to shed light on the quest to understand the relation of particle production at the QCD phase boundary.

As the coalescence process for the formation of anti-hypernuclei requires that anti-nucleons and anti-hyperons are in proximity in phase space, anti-hypernucleus production is sensitive to the correlations in phase-space distributions of nucleons and hyperons. Equilibration between the strange quark flavour and light quark flavours is one of the proposed signatures of QGP formation, which would result in high anti-hypernucleus yields. In addition, recent theoretical studies motivate a search for the onset of QGP by studying the evolution of the baryon – strangeness correlation as a function of the collision energy. The 3 ΛH yield, for example, provides a natural and sensitive tool to extract this correlation, as it can be compared to the yields of 3 He and 3H, which have the same atomic mass number.